http://www.technologyreview.com/news/428477/the-iphone-has-passed-a-key-security-threshold/
Technology Review, published by MIT, says:
...when iOS devices are turned off, the copy of the encryption key in the computer's accessible memory is erased. That is why an investigator who gets a suspect's phone would have to try all possible keys—the task deemed impossible by the NSA.The iPhone and iPad do keep a copy of the encryption key deeper in flash memory—otherwise there would be no way for the device to recover data when it was turned back on. But that encryption key is itself protected by the user's "PIN lock," a code that must be entered before the device can be used.
...if the user chooses a six-digit PIN, the maximum time required would be 22 hours; a nine-digit PIN would require 2.5 years, and a 10-digit pin would take 25 years. That's good enough for most corporate secrets—and probably good enough for most criminals as well.
Google's Android operating system also supports encrypted storage, but only for some of the data on the phone. More important, there is no key burned into the hardware, so even complex passwords can be broken by extracting them and using a network of a few hundred computers.
